Cryogenic separation of air requires a pre-purification step to remove contaminants such as water, CO2 and hydrocarbons from air. In cold sections of the separation process (such as heat exchangers and LOX sump), water and CO2 can solidify and block the heat exchangers or other components in the distillation columns. Acetylene and other hydrocarbons in air present a potential hazard. The high boiling hydrocarbons can accumulate in the liquid oxygen and create an explosion hazard. Thus, those impurities in air must be removed in an adsorptive clean-up process prior to the cryogenic distillation of air.
Nitrous oxide (N2O) should also be removed from air prior to separation. N2O is currently present in air at a concentration of about 300-350 ppb, however, this concentration is increasing annually at a rate of about 0.3%. Various factors such as emissions from motor vehicles, HNO3 plants, adipic acid and caprolactam plants (both use HNO3 for oxidation of inorganics) contribute to this growing ambient concentration of N2O. The presence of greater than 50 ppb of N2O can be a serious problem for cryogenic air separation units (ASU) because it can form solid deposits in distillation columns. N2O also decreases the solubility of CO2 in liquid oxygen, thereby increasing the potential for freezing of CO2 in the distillation columns. This can result in degraded performance and can even cause blockage of heat exchangers.
Air prepurification can be accomplished using pressure swing adsorption (PSA), temperature swing adsorption (TSA) or a combination of both (TSA/PSA) incorporating either a single adsorbent or multiple adsorbents. When more than one adsorbent is used, the adsorbents may be configured as discrete layers, as mixtures, composites or combinations of these. Impurities such as H2O and CO2 are commonly removed from air using two adsorbent layers in a combined TSA/PSA process. Normally, a first layer of activated alumina is used for water removal and a second layer of 13x molecular sieve is used for CO2 removal. Prior art, such as U.S. Pat. No. 4,711,645, teaches the use of various adsorbents and methods for removal of CO2 and water vapor from air.
Centi et al. (Ind. Eng. Chem. Res., vol. 39, pp 131-137, 2000) studied the behavior of various ion exchanged forms of ZSM5 Zeolites for removal of relatively high concentrations of N2O (500 ppm (parts per million) to 2000 ppm) from industrial gas streams. ZSM5, being a high Si/Al ratio (2-200) zeolite, has less water affinity than its low Si/Al ratio counterparts. The best performance for N2O removal in Centi's study is shown by Ba and Sr exchanged ZSM5. The paper indicates that in the presence of water, metal exchanged ZSM-5 has better N2O adsorption properties than lower Si/Al ratio zeolites such as X and Y type zeolites.
U.S. Pat. No. 6,106,593 teaches a process, preferably TSA, that uses a three-layer adsorbent bed for successive removal of water, CO2 and N2O, wherein the preferred adsorbent is binderless CaX. Other adsorbents such as CaX (with binder), BaX and Na-mordenite are also recommended for the third layer. According to the patent, the criteria for selecting an adsorbent for N2O removal is a Henry's law selectivity for N2O compared to CO2 of 0.49 or more at 30° C. and a Henry's law constant for N2O adsorption of at least 79 mmol/gm.
European patent application EP 0 862 938 teaches the placement of a zeolite adsorbent selected from X-zeolite, Y-zeolite, A-zeolite or mixtures thereof downstream of an alumina adsorbent in a PSA process to remove nitrogen oxides, such as NO, NO2, N2O and N2O3. European Patent Application EP 0 995 477 teaches a method of removing at least a portion of N2O in a gas stream using a type-X zeolite with a Si/Al ratio of 1.0-1.5 and containing a mixture of K+(<35%), Na+(1-99%) and Ca2+(1-99%) cations in various proportions.
European Patent Application (EP 1 092 465) teaches a TSA process (sequentially removing H2O, CO2 and N2O and optionally hydrocarbons using a three-layer configuration of adsorbents. A NaLSX adsorbent is preferred in the second layer for CO2 removal. A LSX zeolite (Si/Al=0.9-1.3), preferably CaLSX zeolite, is suggested for N2O and hydrocarbon removal.
European Patent Application EP 1 064 978 teaches the use of BaX zeolite to remove propane, ethylene and N2O in a PSA or TSA process. The BaX zeolite contains at least 30% barium cations.
U.S. Pat. No. 4,156,598 teaches the method of removing N2O from nitrogen trifluoride by passing the gas through a synthetic zeolite adsorbent, such as sodium or calcium exchanged type X or type A zeolite.
U.S. Pat. No. 4,933,158 teaches a method of removing N2O and CO2 from nitrogen trifluoride by passing the gas through a thermally treated zeolite selected from the group consisting of analcime, clinoptilolite, mordenite, ferrierite, phillipsite, chabazite, erionite and laumotite.
U.S. Pat. No. 4,507,271 teaches the method of removing N2O from a gas containing hydrogen, nitric oxide and nitrous oxide using A, X or Y zeolite.
U.S. Pat. No. 5,587,003 discloses a method for removing substantially all of the CO2 from air using the adsorbent clinoptilolite.
Rege et al. (Chemical Engineering Science, vol. 55, pp 4827-4838, 2000) showed 13x adsorbent to provide better CO2 removal from air than clinoptilolite. Rege also showed that Ca-exchanged clinoptilolite to have low N2 adsorption.
Catalytic decomposition of the contaminant is another means of removing an undesirable component from a gas mixture. A catalyst/adsorbent can be used much in the same way as described above except that the product of decomposition must be either removed as an additional contaminant or be an acceptable component of the gas mixture.
The prior art has typically derived its solution to the problem by seeking adsorbents with high N2O to CO2 selectivity. However, given the similar electronic structure of N2O and CO2, and the nearly 1000-fold difference in gas phase concentration between N2O and CO2 in air, this methodology is difficult to apply. Thus an improved process and apparatus for the removal of N2O and other impurities from air is required.